Investigators for special counsel Robert Mueller have been asking witnesses about President Donald Trump's business activities in Russia before the 2016 campaign as he considered a White House bid, CNN reported.

According to the outlet, which cited unnamed sources, questions have included the timing of Trump's decision to seek the presidency, potentially compromising information the Russians might have had about him, and why efforts to brand a Trump Tower in Moscow fell through.

"You ask everything even if you don't think it's credible," one source told CNN, adding, "the allegations are out there, and it was checking the box."

In an interview with The New York Times last July, Trump said he would view any investigation of his or his family's personal finances as a "violation" by Mueller that crosses a red line.

Trump's lawyers previously have said Trump's business dealings from the time before he was a presidential candidate do not fall under the scope of what Mueller is authorized to investigate by the Justice Department, CNN noted.